Using current technology the transmission distance of fiber optic communication systems is several hundreds of kilometers at a 2.5 Gbit/s modulation rate. Speed constraints associated with electronic components and wavelength chirp induced errors pose limitations on achieving higher data rates using modulated optical transmitters. Nevertheless, there is an ongoing requirement to increase the transmission bandwidth in order to allow the delivery of video and high density computer data. It is generally recognized that wavelength division multiplexing (WDM) is an effective technology to increase the capacity of future systems by exploiting the bandwidth capability in single mode fibers and/or erbium doped fiber amplifiers (EDFA). The bandwidth potential of single mode optical fibers is in the terabit range. In order to effectively implement WDM technology, a plurality of wavelength channels are required wherein each channel has the capability of being independently modulated with distinct data trains.
The optimum frequency band for transmission via a single mode optical fiber is relatively narrow taking into account dispersion and attenuation characteristics. Accordingly, it is advantageous to segment the useful transmission window into as many individual channels as possible. This requires transmitters that can be selected or set to emit at exactly a predesigned wavelength. Semiconductor lasers and particularly lasers fabricated from III-V alloys can be tailored to emit in the 1250 nm to 1600 nm wavelength range, the range best suited for single mode optical fibers. A III-V alloy of particular interest is the InGaAsP/InP system which can be used to fabricate lasing devices having an optical output within this range.
It is known, of course, to select discrete laser devices each having a given wavelength and to assemble hybrid arrays of such devices in order to provide a range of discrete channels. Such a process, however, is not cost effective as it requires extensive pre-selection to locate lasers of specific wavelength. Considerable assembly time is also required and extra packaging is necessary. Additionally, the accuracy with which discrete devices may be individually mounted on a carrier is inferior to that which may be attained by monolithic processing equipment. Such hybrid structures also tend to be inferior with respect to stability and reliability.